This invention relates to improvements in the combustion chamber of compact engines of the side valve type for universal use incorporating a contactless ignition system having outstanding firing performance.
It is well known that ignition systems presently used in gasoline engines include a magneto of the contact type by which the high voltage obtained by electromagnetic induction is applied to an ignition plug for spark discharge to fire an air-fuel mixture. In recent years, however, attention has been directed to ignition systems of the contactless type, and more particularly to the capacitor discharge ignition system (hereinafter referred to as "CDI system"). In fact there is a growing tendency to employ the CDI system.
The CDI system is operable free of the drawbacks of the contact-type magneto, especially the troubles due to staining of the contact points. Since the system does not involve arcing which occurs across the points of a contact-type magneto, the system gives substantially constant secondary voltage for low-speed to high-speed operation, exhibits outstanding starting and low-speed performance without mis-sparking and is serviceable almost permanently free of point failures because the system is of the contactless type in which electronic signal causes sparking. Because the system does not include the points and lifter which are required for contact-type magneto and which are subject to abrasion, the system assures accurate ignition timing at all times and therefore retains good performance for a prolonged period of time.
The CDI system has another advantage that the high primary voltage permits the secondary voltage to build up quickly, consequently firing the ignition plug with a large spark without any failure.
Despite the various advantages described above, the CDI system remains yet to be improved for use in compact engines of the side valve type as will be described below. Because of the characteristics of the CDI system, the sparking time (50 to 100 microseconds) is 1/20 to 1/10 of that of the contact-type magneto, thus failing to satisfactorily establish steady combustion in conventional side valve type compact engines. The system is not properly adapted for combustion chambers designed for the conventional contact-type magneto. In fact when the system is incorporated into compact engines of the side valve type, misfiring frequently occurs especially during no-load running, producing irregular combustion noises and giving discomfort to the operator. These drawbacks may be overcome by prolonging the sparking time of the ignition plug, but this renders the system large-sized and expensive, with the result that it becomes difficult to install the system in compact engines in view of the limited space available for installation and cost.